1. Field of the Invention
The present invention relates to a liquid crystal display device and more, precisely, to such device comprising an active substrate with a pixel transistor, TFT, thereon and a counter substrate facing the active substrate via liquid crystal therebetween.
2. Description of the Related Art
A liquid crystal display device has heretofore been known, which comprises an active substrate as prepared by forming a TFT (thin film transistor) on a substrate of, for example, glass or quartz, and a counter substrate that faces the active substrate via liquid crystal therebetween.
As a rule, the device of that type receives the rays from a light source through its counter substrate. The rays, if entering the pixel transistor in the device, will deteriorate image quality through contrast depression or flickering owing to photoelectric current leakage.
The sensitivity of polycrystalline Si (poly-Si) is not as high as compared with that of amorphous Si (a-Si). However, recent liquid crystal display devices with poly-Si-TFTs are often used in the presence of a large quantity of light, for example, as in projectors, and photoelectric current leakage is not negligible even in those devices with poly-Si-TFTs. Therefore, with the devices with poly-Si-TFTs, the problem of image quality deterioration through contrast depression or flickering owing to photoelectric current leakage comes into question.
One related technique of preventing ray penetration into the pixel transistor through the counter substrate comprises disposing a black matrix B adjacent to the counter substrate 2, as in FIG. 1A. As illustrated, the black matrix B acts to shield the pixel transistor from the incident rays. In this structure, however, it is impossible to prevent a part of the incident rays L2 having scattered or reflected, from entering the pixel transistor 7, even though the straight incident rays L1 could be blocked by the black matrix B. To overcome this problem, the present inventors previously proposed a technique of disposing a black matrix above the transistor on the side of the active substrate 1 facing the counter substrate 2, but not just below the counter substrate 2 to be adjacent thereto, as in FIG. 1B. In this structure, the black matrix is nearer to the transistor, by which the incident rays going into the transistor could be reduced more effectively (see Japanese Patent Laid-Open No. 262494/1996). As illustrated in FIG. 1B, the incident rays L2 having scattered or reflected could be prevented from entering the pixel transistor 7. In this proposal, the black matrix is in two sites, both acting as a light-shielding layer.
As shown, however, a part of the passing rays L3, having reflected in the optical system, produces return rays (stray rays) that enter the transistor through the active substrate. In any structure, it is impossible to prevent the return rays (stray rays) L3 from entering the transistor part 7.
In particular, in a liquid crystal display device with a top-gate-structured or planar-structured poly-Si-TFT, the active layer of the transistor is formed on the active substrate to be the lowermost layer thereon (opposite to the counter substrate). In this, therefore, the rays running toward the active substrate directly enter the active layer of the transistor, thereby causing photoelectric current leakage.